Amadeus VIII, byname Amadeus The Peaceful, Italian Amedeo Il Pacifico, French Amédée Le Paisible, also called Pope Felix V (born 1383, Chambéry, Savoy [now in France]—died Jan. 7, 1451, Geneva [now in Switzerland]) count (1391–1416) and duke (1416–40) of Savoy, first member of the house of Savoy to assume the title of duke. His 42-year reign saw the extension of his authority from Lake Neuchâtel on the north to the Ligurian coast, and under the title of Felix V he was an antipope for 10 years (1439–49).

The sudden death of Amadeus VII in 1391 left his eight-year-old son under a regency. In 1393 Amadeus married Mary, daughter of Philip the Bold, duke of Burgundy. During the early 1400s he increased the territories of Savoy, laying claim to Geneva in 1401 and buying other nearby lands in 1402 and 1406. In 1416 the Holy Roman emperor Sigismund elevated the county of Savoy to the rank of a duchy.

In 1434, after 42 years on the throne, Amadeus retired to a monastery at Ripaille. His retirement was only partial, however, and he continued to exercise power, with his son Louis (Ludovico) acting as his lieutenant. During that period Amadeus’ daughter Margherita was betrothed to Louis III of Anjou, pretender to the throne of Naples. Louis III died suddenly in 1434, and Amadeus briefly claimed Naples for Margherita but in the end abandoned the kingdom to Alfonso V of Aragon. In 1440 Amadeus abdicated.

One adventure remained to the aging duke: when Pope Eugenius IV broke off relations with the rebellious Council of Basel in 1439, the mutinous prelates elected Amadeus pope under the name of Felix V. Amadeus-Felix resigned 10 years later, under pressure from the kings of France, England, and Sicily, and was a cardinal for his last two years. He was the last of the antipopes.

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...suspended Eugenius, he excommunicated its members. The council, with only seven bishops present, then declared Eugenius deposed and in 1439 elected as his successor a layman, the Duke of Savoy, Amadeus VIII, who took the name Pope Felix V. The next 10 years of this rump council are important only because the princes used it to strengthen their control over the churches in their own...